The human vertebral column (or “spine”) connects the skull to the pelvis and includes a column of vertebrae, the sacrum, intervertebral discs and the coccyx. The vertebrae, which are the bones of the spine, function to bear the weight of the body and to house the spinal cord and spinal nerve roots within the vertebral column. There are two major parts to each vertebra: (1) the anterior (front) segment, which is the vertebral body; and (2) a posterior part (the vertebral arch) which encloses the vertebral foramen. The vertebral arch includes a pair of pedicles, a pair of laminae, and seven processes, four articular, two transverse and one spinous. The transverse processes are relatively long and slender while the spinous processes are relatively broad and thick. The processes provide sites for the attachment of ligaments and muscles which are important for the stability and movement of the spine.
Intervertebral discs lie between adjacent vertebrae in the spine. Each intervertebral disc includes a soft jelly-like center called the nucleus pulposus, which is surrounded by the annulus fibrosis, which includes several layers of fibrocartilage. The nucleus pulposus acts as a shock absorber for the spine, absorbing the impact of the body's daily activities and keeping the adjacent vertebrae separated. The nucleus pulposus is predominately made of water, which gives the intervertebral disc its elastic quality. However, as people age, the nucleus pulposus begins to dehydrate, which limits its ability to absorb shock and separate the adjacent vertebrae. This disc degeneration can result in spinal stenosis, a medical condition in which the spinal canal narrows, thereby producing pressure on the nerve roots resulting in pain and discomfort. Spinal stenosis can also be caused by spinal disc herniation, osteoporosis, or a tumor. Multiple different medical procedures have been developed to help alleviate the pain and discomfort associated with spinal stenosis.
One medical procedure that has been developed to alleviate the pain and discomfort associated with spinal stenosis includes distracting the spinous processes of adjacent vertebra using an interspinous implant. The interspinous implant can be placed between the spinous processes to increase the minimum distance between the spinous processes that occurs during extension motion, while substantially retaining flexion motion, axial rotation and lateral bending. The use of an interspinous implant may, however, place an unnatural amount of stress on the spinous processes supporting the interspinous implant. This can be problematic, especially when the spinous process has been weakened by old age and/or another medical condition (such as by osteoporosis). A weak spinous process may fracture or crack when using the implant. It can be desired to increase the strength of a spinous process, thereby allowing a wide range of interspinous implants to be used to distract the spinous processes of adjacent vertebra.